The present invention relates generally to providing electrical power to a plurality of low voltage electrical loads, and, in particular, to a string of Christmas lights.
In FIG. 1, light set 10 is a standard string of lights that is currently in widespread use. Light string 10 is powered by inserting a standard plug 12 into a wall outlet (not shown). The lamps 14 in light set 10 are arranged in a series electrical circuit. This configuration is the least expensive circuit for a string of lights, that is, for a plurality of low-voltage, low-current, small-sized electrical loads. Depending on the number of lamps, say 50 in a typical string, each lamp may typically require 2.5 volt at 200 milliamps. in a series configuration, the set then requires 120 volts to light it.
There are larger sets of lights, using 100 and 150 lights in a string. However, these are typically composed of three strings of 50 lights each, each string arranged electrically in a parallel circuit with each other string and each lamp within a string in series electrically with each other lamp in that string.
Normally, in a series circuit, when one bulb burns out, the set will not light until that bulb is replaced. Each lamp in these longer circuits, however, is equipped with a shunt that continues to pass the electric current around the bulb in the event that bulb burns out. The shunt is an aluminum oxide wire that is wrapped around a filament standoff post. When the filament burns open and current cannot flow through it, there is suddenly no voltage drop across the set. Then the voltage across the lamp rises quickly to line level (120 volts), arcs across the insulated shunt and welds the shunt across and into the circuit of the bad lamp so that current once again begins to flow through the light set. Although the lights in the light string are once again lighted, each bulb carries slightly more voltage because of the low-load shunt in the burned out bulb.
In the event that the shunt fails to save the light string, which happens about 30% of the timexe2x80x94higher in older light setsxe2x80x94the light string will fail completely.
In fact, in most instances, the failure of the light string is not caused by a bulb burning out but by a failure between the bulb contacts and the contacts in the socket the bulb is received in. The contacts are typically a nickel copper alloy and the socket contacts are made of brass (a tin/copper alloy). The contact between these dissimilar alloys will react in a Galvanic manner, degrading their contacting surfaces and thus the quality of the electrical connection between contacts until current flow is stopped and the lights go out.
Thus when a bulb is missing or its contacts are degraded sufficiently, the string will fail to light. In a string of 150 lights, this is a tedious problem to remedy.
Arranging the lights in parallel is not the answer although the string would light if one light were defective or missing or its contacts were degraded. A standard Super Bright lamp consumes 200 milliamps of power, a set of 150 lamps would draw 30 amps of power at 120 VAC, or 3600 watts, far too much power, and also a fire hazard for use as a decoration for a Christmas tree.
Consumption of power is a significant problem not only with a hypothetical string of 150 parallel lights but also with light strings generally. A 150 lamp conventional string will consume 72 watts of power and there are typically between 400 to 600 lights used on a single tree.
The use of both parallel and series configured bulbs in one string of Christmas tree lights is not unknown. For example, Smith et al, in U.S. Pat. No. 4,675,575, describe a light-emitting diode (LED) assembly for lighting a tree. Their strings of LEDs can be used with AC or DC. However, LEDs do not require much power and do not produce much light compared to incandescent bulbs, regardless of how small the latter are, and therefore do not have the inherent limitations of more conventional Christmas tree lights.
Mancusi, Jr., in U.S. Pat. No. 4,855,880, teaches a different arrangement of lights on a light string for illuminating a Christmas tree. His string includes incandescent xe2x80x9cseedxe2x80x9d bulbs arranged electrically in series and in parallel in an artificial tree. Rectifying conventional AC with a selenium rectifier to power his lights, he combines in series up to twelve sets of forty lights each; each light in a set is in parallel.
Also, Crucefix discloses another light system configuration in U.S. Pat. No. 4,870,547, based on a collar that is placed around the trunk of the tree at its top and which is used to deploy the various parallel sets of series lights.
There remains a need for an effective, low-power electrical circuit for a Christmas light string, or indeed, for any set of plural low voltage loads.
According to its major aspects and briefly recited, the present invention is an electrical circuit for a plurality of low-voltage electrical loads such as a string of Christmas lights. The circuit comprises groups of lights arranged in an electrical series circuit, and each light within a group is arranged in parallel. Preferably, five lights comprise each group and there are thirty groups to form a string of 150 lights, matching the number of lights in presently-available light strings. By varying the lamp voltage and the number of groups, light strings could range in size from 50 to over 200, matching and exceeding current popular sizes. Parallel to each group is a series of semiconductors or bipolar devices forming a parallel group device that limits voltage and current in the event one or more of the lights in that group goes out.
The present circuit can be used with any AC plug but is preferably used with the DC plug described in commonly owned U.S. Pat. No. 5,777,868.
A major advantage of the present invention is reduced power consumption. By comparison, instead of the 72 watts of power used by conventional strings of 150 lights, the present string uses only 10.8 watts of power, when using the DC plug recited in the co-pending application.
Another major feature of the present invention is the ability to achieve the low current and voltage advantages of a series light string while not allowing the loss of one bulb to cause a failure in the entire string. In one embodiment, with a semiconductor series or bipolar device arranged parallel to each group, loss of all the bulbs in the same group would not cause the string to fail. This arrangement eliminates a major problem of having one bulb produce the failure of the entire string as with series groups while preventing the failure of a single bulb to create a current stress on remaining bulbs as in parallel groups. In a normal parallel light string connected in series with other parallel light strings, after one bulb burns out, the other bulbs in that parallel group are exposed to a greater current level which in turn creates failures in more bulbs, consequently, these failures create a greater current stress in the remaining bulbs in that parallel group until all bulbs fail at an exponential rate. Moreover, with each failure, the light string produces a higher level of heat which shortens lamp life and produces a fire hazard.
In contrast, the present invention eliminates this avalanche effect by providing a semiconductor series or bipolar device that regulates current so that the remaining lamps are not stressed by additional current.
Other features and their advantages will be apparent to those skilled in the electrical arts from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the drawings.